CSI Wireless GBX, Reference Manual

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GBX Reference Manual
2
1.1.2 GPS Services
The positioning accuracy offered by GPS varies depending upon the
type of service and equipment available. For security reasons, two
GPS services exist: the Standard Positioning Service (SPS) and the
Precise Positioning Service (PPS). The US Department of Defense
(DoD) reserves the PPS for use by its personnel and authorized
partners. The DoD provides the SPS free of charge, worldwide, to all
civilian users.
In order to maintain a strategic advantage, the US DoD artificially
degrades the performance of the SPS so that the positioning accuracy
is limited to 100 meters 95% of the time. This intentional degradation
is called Selective Availability (SA).
For many positioning and navigation applications, an accuracy of 100
meters is insufficient, and differential positioning techniques must be
employed.
1.2 Differential GPS
The purpose of differential GPS (DGPS) is to remove the effects of SA,
atmospheric errors, timing errors, and satellite orbit errors, while
enhancing system integrity.
1.2.1 How it Works
DGPS involves setting up a reference GPS receiver at a point of known
coordinates. This receiver makes distance measurements, in real-
time, to each of the GPS satellites. The measured ranges include the
errors present in the system. The base station receiver calculates
what the true range should be, without errors, knowing its coordinates
and those of each satellite. The difference between the known and
measured range for each satellite is the range error. This error is the
amount that needs to be removed from each satellite distance
measurement in order to correct for errors present in the system.